1. Field of the Invention
The present invention relates to a process for interception-protected frequency band compressed transmission of speech signals in accordance with the channel vocoder principle, wherein, at the transmitting end, in an analysis component, sample values indicating the mean spectral power and sample values representing other specific speech parameters are obtained in each analysis interval via rectifiers from the signal energy which is split into a plurality of spectral channel, which sample values are transmitted to the receiving end, together with at least one item of synchronizing information in the form of a time-division multiplex (TDM) frame (sum signal), and wherein, at the receiving end, the individual sample values of the TDM subsignal are distributed frame-by-frame amongst associated spectral channels and amongst associated inputs for the specific speech parameters and the synchronizing information of a synthesis component which synthetically generates the original speech signal therefrom.
2. Description of the Prior Art
Processes of the type set forth above are known, for example through the German published application No. 11 73 948 and are used wherever a transmission channel of adequate bandwidth is unavailable for conventional speech transmission. When the channel vocoder principle is used, it is possible to transmit a speech signal across channels whose transmission bandwidth is only 1,000 Hz and less. As only the envelope curve values of the spectral channels and other specific speech parameters, such as the fundamental speech frequency and the picture signal are transmitted in the case of the channel vocoder, a transmission channel of this kind exhibits good resistance to interception simply for this reason.
If higher requirements are placed on the transmission of speech with regard to its resistance to interception, the normal practice is to convert a speech signal of this type into a coded speech signal and to encode the coded signal by the use of a modulo-2 addition with a code sequence. Here again, the use of the channel vocoder principle offers considerable advantages with regard to the least possible bandwidth requirement. In place of the 64 kbit normally required in this case, it is possible to transmit a digital channel vocoder signal in encoded form even at 2.4 kbit/sec. For this purpose, the sample values of the spectral channels, which have been determined in the analysis component, and sample values representing other predetermined speech parameters are coded prior to their composition to form a TDM sum signal, and the TDM digital sum signal is subsequently encoded in the conventional manner using a modulo-2 addition with a code sequence.
As indicated by practice, the transmission of digital, and possibly encoded channel vocoder signals, by shortwave radio gives rise to considerable difficulties due to multiple path propagation and due to selective fading. In the case of radio frequency signals in which the speech signal, in analog form, is modulated onto a carrier with amplitude or frequency modulation, the disturbing influence of multiple path propagation and of selective fading generally only leads to signal distortions which can be largely eliminated, for example, by emphasizing the transmitted level. On the other hand, in the case of digital radio signals, for example, transit time differences which arise with multiple path propagation in the order of one bit are in themselves sufficient to lead to a complete breakdown of the transmission length. Neither can the interference be counteracted in this case by emphasizing the transmission level. Moreover, the disturbing effect of multiple path propagation is the greater, the higher the selected bit rate within a channel. However, due to the digitization of the speech signal, when the channel vocoder principle is used, the bit rate cannot be substantially reduced below 2.4 kbit/sec. When four-phase modulation is used, for example, a bandwidth of approximately 3 kHz is needed.